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Vapor lock with deadhead regulator on heat soaked motor

Discussion in 'Tech Discussion' started by Mr. Goodtool, Mar 26, 2020.

  1. Mr. Goodtool

    Mr. Goodtool Junior Member

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    I have a 3 port mechanical fuel pump on my 406 chevy small block. If I change to a 2 port pump, and change my regulator to a by-pass style, then run the by-pass line from the new regulator back to the third vent return line that goes back to the tank, will this fix my vapor lock problem?? Anybody try this? Is this workable??

    truck engine compartment.JPG
     
  2. Dutch Rutter

    Dutch Rutter Full Access Member

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    I had a similar issue myself on hot starts. I ended up adding a 3 port regulator and blocked the return port on the regulator and kept the 3 port pump to return fuel as needed. Works well for my edelbrock carb and solved the issue. Either way would work as long as you have some kind of return for the excess fuel.
     
  3. Mr. Goodtool

    Mr. Goodtool Junior Member

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    Hi Dutch! Thanks for your reply! The picture of my engine compartment is kinda old, in the sense that the fuel routing has changed. The fuel line to the carb, goes through a filter first, then into a dead head regulator (non-by-pass type) then on to the carburetor...A Jet performance, 850 cfm quadrajet. The regulator is plumbed with a pressure gauge on one of the two outlet ports and the other side feeds the carb. On very hot summer days, it always starts fine, but when stopped at signal lights for the duration, when I accelerate into the intersection, the motor will stumble and surge until the flowing extra air cools down the engine compartment. I need to know how to get rid of excess vapor, that blocks fuel flow. That's why I'm hoping a by-pass regulator will solve my dilemma.
     
  4. CoggedBelt75

    CoggedBelt75 Full Access Member

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    I’m no mechanic, but seems to me if you were experiencing vapor lock it would die and wouldn’t restart until things cooled off. At least, in my cases, that was what happened. Stumbling could be one of several things. My setup is the usual supply line in, then out of the pump, thru a filter, then into the regulator into the carbs. No return line to the tank. Big heat pump on top of the engine. Vapor lock never has been an issue in our hot summers.
    upload_2020-3-26_19-33-54.jpeg
    Hope you find your problem and if so, report the fix.
     
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  5. highdesertrange

    highdesertrange Full Access Member

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    a return line only works if the engine is running. once you shut it off and fuel stops flowing all bets are off. highdesertranger
     
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  6. Matt69olds

    Matt69olds Full Access Member

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    The return line and 3 port pump is designed to circulate fuel to keep the pump cool. Once the engine shuts off, the return line gives the boiling fuel a place to vent pressure buildup. If the return line is restricted or plugged up, the heat from the engine will make the fuel boil, raising the pressure high enough to push open the float needle, causing the engine to flood. That’s the biggest reason cars/trucks with A/C got the fuel pump with return. If you have a3 port pump, and the return is clear, I’d look elsewhere for your problem before messing around with regulators.
     
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  7. mike broussard

    mike broussard Full Access Member

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    I agree with Cogged..It doesn't sound like vapor lock.
    I'm curious about your float bowl level though.
    Have you always had this problem?
     
  8. Dutch Rutter

    Dutch Rutter Full Access Member

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    I'm with clogged belt as well, doesn't seem quite right. I know that edelbrocks dont like to see much more then 6psi. But I dont know if qjets have that same issue. If it were mine I guess I'd be thinking a fuel starvation issue, or a different carb issue (along with what else has been said). Wouldn't hurt to try it with the regulator bypassed and see what it does. I'm also no expert just throwing out ideas.
     
  9. Rusty Nail

    Rusty Nail Whatever!

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    Does the symptom duplicate without a gas cap?
    Verified spark?
     
  10. CoggedBelt75

    CoggedBelt75 Full Access Member

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    That’s just not vapor lock as I know it. I usually don’t respond to these kind of posts but thought I would state the obvious. Seems like people need help on stuff but are vague is describing all the symptoms. And it seems they also usually forget to mention they “messed” with it or what they did when doing so. Denis posted over 24 hrs ago and hasn’t came back to say what’s going on. Maybe he found the fix. Or y’all wasn’t quick enough with the answers:p
     
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  11. mike broussard

    mike broussard Full Access Member

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    Yep, lack of details really makes it tough to help. As well as him thinking it was vapor lock without supporting info.
    My guess is the float is letting too much fuel in to the bowl at idle. It clears up once the carb starts pulling more fuel, but again we have such limited information. It could almost be anything.
     
  12. Craig Nedrow

    Craig Nedrow Full Access Member

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  13. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

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    I’ve been having some vapor lock issues on anything resembling a warm day. I get a little bit of stumble in motion, but it’s mostly heat soak related fuel boil/vaporization.

    The phenolic spacer seems like a good idea to prevent heat conduction, but the Q-Jet carb gasket is pretty thick, and I question how much real world improvement would be yielded there. I’m trying ethanol free 87 and seeing how much improvement I get there. Ethanol’s boiling point is considerably lower than pure gas. Plus, my system predates factory return on GM passenger cars even though I have A/C and use it religiously so I’m already at a loss there.

    If I’m unsuccessful, I might look into making a carb blower fan that has access to ambient air via the Thermac. It’d be relatively easy in a square that has fresh air access courtesy of a duct, not as much a car whose snorkel sits under the hood. MG and Toyota, IIRC, toyed with carb fans back in the day, but their setup is carb proximal, about where your dashpot would sit and sources air from well inside the engine bay. GM tried one on F Bodies, but it sat on the fender, had independent ducting, and ran from a constant hot through a temp switch to try and cool the carb down to 165*. External booster pumps have seen some success in the mid-80s P30 based motorhomes, but in-tank booster pumps were more reliable at the tail end of carburetion.

    GM did a bunch of vapor lock related service bulletins targeted towards F Bodies, but they apply directly to B and G Bodies and indirectly to and I need to go back peruse those closely. To anyone interested, I would suggest checking those out.

    https://www.thirdgen.org/forums/carburetors/320143-vapor-lock-bulletins.html
     
  14. highdesertrange

    highdesertrange Full Access Member

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    "Vapor lock can be cured with one of these REAL phonolic spacer under the carb."

    only if the issue is the carb. vapor lock can happen anywhere along the fuel line or at the pump or at the carb.

    highdesertranger
     
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